In recent years, as semiconductor integrated circuits (hereinafter “LSI”) are made high-integration and high-performance, new techniques for fine processing have been developed. Chemical-mechanical polishing (hereinafter “CMP”) is also one of them. The CMP is often used in LSI fabrication steps, in particular, in making interlaminar insulating films flat in the step of forming multilayer wirings, in forming metallic plugs and in forming buried wirings. This technique is disclosed in, e.g., U.S. Pat. No. 4,944,836.
Recently, in order to make LSIs high-performance, it is also attempted to utilize copper alloys as wiring materials. It, however, is difficult for the copper alloys to be finely processed by dry etching often used in forming conventional aluminum alloy wirings. Accordingly, what is called the damascene method is chiefly employed, in which a copper alloy thin film is deposited on an insulating film with grooves formed previously and is buried therein, and the copper alloy thin film at the part except the grooves is removed by CMP to form buried wirings. This technique is disclosed in, e.g., Japanese Patent Application Laid-open No. 2-278822.
In a common method of polishing metals by the CMP, a polishing pad is fastened onto a circular polishing roller (platen), and the surface of the polishing pad is soaked with a polishing medium for chemical-mechanical polishing, where a substrate with a metal film formed thereon is pressed against the pad on the former's metal film side and a stated pressure (hereinafter “polishing pressure”) is applied thereto on the back thereof, in the state of which the polishing platen is turned to remove the metal film at the part of its hills by mechanical friction acting between the polishing medium and the metal film.
The polishing medium used in such CMP is commonly comprised of an oxidizing agent and solid abrasive grains, to which a metal-oxide-dissolving agent and/or a protective-film-forming agent are optionally added. The basic mechanism of CMP making use of this polishing medium for CMP is considered to be that the metal film surface is oxidized with the oxidizing agent and the resultant oxide layer is scraped with the solid abrasive grains. The metal surface oxide layer at the part of dales does not come into contact with the polishing pad so much and the effect of scraping attributable to the solid abrasive grains does not extend thereto. Hence, with progress of the CMP, the metal layer becomes removed at its hills and the substrate (with film) surface become flat. Details on this matter are disclosed in Journal of Electrochemical Society, Vol. 138, No. 11 (published 1991), pages 3460-3464.
In order to make higher the rate of polishing by CMP, it is considered effective to add the metal-oxide-dissolving agent. It can be explained that this is because the effect of scraping attributable to the solid abrasive grains comes higher where grains of metal oxide scraped off by the solid abrasive grains are made to dissolve in the polishing medium. However, the addition of the metal-oxide-dissolving agent makes the metal film surface oxide layer dissolve (hereinafter “etching”) also at the part of dales, and the metal film surface becomes uncovered, so that the metal film surface is further oxidized by the oxidizing agent. With repetition of this, the etching of the metal film may proceed at the part of dales, resulting in a damage of the effect of flattening.
In order to prevent this, the protective-film-forming agent is further added to the metal-polishing medium for CMP. Thus, adding the metal-oxide-dissolving agent and protective-film-forming agent adds the effect of chemical reaction, and this brings about the effect that the CMP rate (the rate of polishing by CMP) is improved and also any injury (damage) of the metal layer surface to be polished by CMP may less occur.
In order to obtain a flat polished surface, it is important to balance the effect attributable to the metal-oxide-dissolving agent and protective-film-forming agent used in the polishing medium for CMP. In the CMP, it is preferable to use a polishing medium which does not etch the metal film surface oxide layer so much at the part of dales, dissolves in a good efficiency the particles of the oxide layer scraped off, and has a high rate of polishing.
However, the formation of buried wirings by CMP using a conventional polishing medium for chemical-mechanical polishing which contain solid abrasive grains involves the problems such that;
(1) it may cause a phenomenon that the surface of the metal wiring having been buried is isotropically corroded at the middle thereof to become hollow like a dish (hereinafter “dishing”);
(2) it may cause polishing mars (scratches) arising from solid abrasive grains;
(3) the wash processing to remove solid abrasive grains remaining on the substrate surface after polishing is troublesome; and
(4) a cost increase may arise because of the prime cost of solid abrasive grains themselves and the disposal of waste liquor.
Accordingly, in order to keep the dishing from occurring or the copper alloy from corroding during the polishing, to form highly reliable LSI wirings, a polishing method making use of a polishing medium for CMP which contains a metal-oxide-dissolving agent comprised of amino acetic acid (e.g., glycine) or amidosulfuric acid and contains BTA (benzotriazole) is proposed. This technique is disclosed in, e.g., Japanese Patent Application Laid-open No. 8-83780.
However, the BTA has a very high protective-film-forming effect, and hence it may greatly lower not only the rate of etching but also the rate of polishing. Accordingly, it is sought to provide a polishing medium for CMP which lowers the etching rate sufficiently but does not lower the CMP rate.